It's possible to clean dust from inside a lens if the dust is behind the front element.
I have a Canon EF-S 17-55mm f/2.8 which is known to be a dust magnet. I know there is dust inside my copy of this lens but they don't show up in my pictures. Other people have cleaned the dust from inside their EF-S 17-55mm f/2.8 and it's a fairly simple process. I'm not sure if it's the same for other lenses though.
Removing Dust From A Canon EF-S 17-55mm